package main

func main()  {
	
}

type lbiao struct {
	val int
	next *lbiao
}

func isExistBack(head *lbiao) bool  {
	c1 := head
	if c1.next == nil {
		return true
	}
	c2 := c1.next
	for c1 != nil && c2 != nil {
		if c1 == c2 {
			return true
		}
		c1 = c1.next
		c2 = c2.next.next
	}
	return false
}